This invention relates to a system and means for placing concrete on the ocean floor at great depths.
Applications for placing concrete in the deep ocean are basically in three areas: in situ construction of anchors and foundations for fixed ocean facilities, in situ hardening of structures or objects on the seafloor, and containment of hazardous or polluting substances for environmental protection. Such applications require portland cement concrete to be placed underwater in quantities of hundreds and thousands of cubic yards in water depths as great as 20,000 feet.
Presently available prior art methods for placing concrete on the seafloor only provide the following capabilities: concrete can be placed on the seafloor or in open forms in water depths to about 400 feet; grouts, which are cement slurries or cement-sand slurries, can be placed underwater in open forms to similar depths; also grouts can be placed underwater at much greater depths, thousands of feed, but only if placed in confined spaces where the flow can be controlled by back pressure, as in an oil well. In general, for most structural applications concrete is superior to grout and costs less. Concrete, which contains larger aggregates than grout, has better structural properties, is heavier and in some cases can be placed without forms which would result in major cost savings.
The most practical way to provide for large holding capacities on the order of 2 to 20 million pounds for fixed ocean facilities in most deep ocean seafloors is to use very large deadweight anchors. In certain hard seafloors, large anchor forces may be best provided by clusters or piles drilled into the bottom and connected together with large pile caps.
A 20 million pound capacity deadweight concrete anchor would have a submerged weight of about 40 million pounds and thus be about 160 feet in diameter by 20 feet thick. This is comparable to the quantity of concrete in a large building mat foundation or a bridge pier but is small compared to a concrete offshore oil platform or a deadweight anchorage for a suspension bridge cable.
Prior methods do not exist for the deployment of large deadweight anchors since the loads are beyond the capacity of existing heavy lift equipment. A number of drill ships exist that can lift about one million pounds in deep water, and a few crane barges are available rated at 6 million pounds for surface or shallow water lifts. One ship in the world, the Glomar Explorer, has had the capability to lift a design load of 8 million pounds from a depth of 17,000 feet. Two or three other recently developed mining ships have deep water lift capacities greater than the drill ships' but much less than the Glomar Explorer's. Outfitting barges or mining ships or re-outfitting the Glomar Explorer for multi-million pound anchor deployment would be very expensive.
A method has been proposed for free-fall emplacement of large deadweight anchors in deep water where seafloor site conditions are favorable. However, this method is designed for applications in which precise positioning is not critical, such as a single point deep ocean mooring but is not appropriate for cases where more precise positioning is important, for example, placing an object at a predetermined seafloor position or placing objects in close proximity to each other. Free-falling is not an appropriate emplacement technique for all sites at which very large deadweight anchors would be used but only for those sites with a soft seafloor at a fairly flat slope.
An alternative to lowering or free-falling a massive anchor is a combination of pre-fabrication and in-situ placement of the present invention as hereinafter described below.
If an object of strategic significance is lost on the seafloor, a decision to salvage the object could be expensive, particularly if the object is lost in deep water and is very large, such as a ship or craft that is too heavy to lift in toto.
Concrete placement makes another option available for consideration. Rather than salvaging the object it can be encapsulated in place on the seafloor by covering it with concrete (FIG. 1). The purpose is to sequester the object in such a way as to deny observation, access or removal of portions of it by others. The operational cost savings for encasing a ship-sized object is substantial compared to a recovery operation.
For many smaller-sized objects, in situ hardening has application where concrete encasement is faster and costs less than recovery.
Another application of hardening is the stabilization of ocean cables and pipelines on firm seafloors in deep water. The purpose is to prevent accidental damage which is caused mostly by trawlers and, importantly, to preclude purposeful damage. At the present time, cables and pipes are protected by burial in those seabottoms soft enough to be trenched. In bottoms not suitable for trenching other protective methods are needed. In some cases, pre-cast concrete covers have been placed over seafloor cables to stabilize them on a firm bottom.
Still another application of placing concrete on the seafloor is to cover or contain hazardous substances for the purpose of isolating them from the environment. Again, this is an alternative to recovery. A hazardous material incident could involve radioactive materials from a nuclear power source. Another example is containment of hazardous materials dumped in the ocean in the past and presenting a potential problem in the present. Leakage problems if they arose could be resolved in many instances by encasement in concrete.
A number of state-of-the-art methods exist for transporting concrete and similar materials by pipeline and for placing them underwater as are discussed briefly, below.
Tremie Method: The construction industry regularly places large quantities of concrete underwater by tremie methods at depths of tens of feet to one or two hundred feet in protected waters for bridge piers and other waterfront type structures. Concrete falls by gravity through open pipes and is placed in forms or confined space. Flow rate is controlled by depth of burial of the lower end of the tremie in the concrete. Good quality concrete is regularly produced using established mix designs and operating procedures. Maximum depth of placement underwater to date is about 400 feet. Major limitations on going deeper are difficulties in starting the flow and maintaining control of the flow without runaway of the high slump concrete in the typically 12-inch or greater diameter pipe. Special approaches have been tried such as foot valves and pipe-within-pipe methods but these do not promise an order-of-magnitude increase in depth capability without considerable development of relatively complex methods. Also, the total weight of tremie pipes filled with concrete becomes very great with increasing depths.
Bucket Method: Large and small quantities of concrete have been successfully placed underwater by covered, bottom-opening buckets of up to several cubic yard capacity. Bucket-placement is used primarily in relatively shallow water although depth is restricted more by operational considerations than by technical limitations. Stiffer concrete, with larger aggregate (up to several inches diameter), can be placed by bucket than by tremie. Specially designed bucket methods have been proposed that would be suitable for placing small but not large quantities of concrete in the deep ocean.
Concrete Pumping: Pumping concrete through pipelines of 2-inch to 8-inch diameter is a well-established practice on land or horizontal distances of 1,000 feet or greater and vertical distances of several hundred feet upward. Reliable equipment and experienced operators are available; mix design is well known to produce pumpable, good quality concrete. Difficulties that do occur are usually due to not following standard procedures, for example, attempting to save costs by using borderline materials, equipment or practices, or are due to operational delays.
Pumping downhill is often troublesome and is not frequently done. However, in some instances, concrete has been pumped down for placement underwater in water depths to about 200 feet. In pumping downhill, it is important to avoid the formation of air pockets and voids in the pipeline. Both large air bubbles and voids can disrupt the flow and cause segregation of the mix which in turn causes blockage of the pipeline. A bleed valve at the high point at the pipeline is used to vent air during initial filling of the pipe with concrete, after which the valve is closed. Flow is then maintained under continuous positive pressure to prevent formation of voids.
Pumping methods offer the potential for an order-of-magnitude increase in water depths at which concrete can be placed provided that means are used to maintain a positive pressure continuously throughout the fully-filled pipe and to control the flow rate, and the characteristics of the fresh concrete required for the controlled flow in the pipeline is compatible with the concrete characteristics required after the concrete is discharged from the pipe at the seafloor. The placement method discussed herein uses a closed system, pumping approach.
Pumping Grouts and Mortars: Grouts is a mixture of either cement and water (neat cement grout) or cement, water, and sand (sand grout), both having a fluid consistency. Mortar is a mixture of cement, water, and sand usually of a stiffer consistency than grout. Grouts and mortars often contain admixtures to control setting, minimize bleeding, or otherwise affect the material characteristics. Grouts and many mortars are readily pumped.
Grouts are regularly pumped through small (e.g., 1-inch) diameter pipes and placed in confined spaces for many construction applications, such as repair of concrete, encasement of post-tensioning tendons, and construction of water cut-off curtains under dams.
Grout pumping is also used for underwater concreting by the preplaced aggregate method by which large quantities of concrete have been successfully placed to depths greater than 100 feet for construction of large bridge piers and other purposes. The coarse aggregate is placed in forms and then intruded with a fluid grout through pre-positioned grout pipes. This method might be adapted to deep ocean placement but probably would require a complex operation since separate placement system would be needed for the forms, the aggregate and the grout. Such a method would be limited to applications using forms or other confined space.
Large quantities of grout, on the order of 10,000 cubic yards, have been placed under offshore gravity-type structures located in water depths to 450 feet. The purpose is to provide uniform bearing on the seafloor and to minimize settlement, especially differential settlement. Grouts used for this purpose develop low strengths and are placed in confined chambers.
Probably the largest deep placement operation was one in which more than 1,300,000 cubic yards of 3/8-inch maximum size aggregate mortar were pumped downward about 1,000 feet into a large water-filled cavity under a dam. The purpose was to fill an enclosed void. Structural grade concrete was not required.
Cementing Oil Wells: Sophisticated above-ground and down-hole equipment, materials and procedures have been developed to cement oil wells to depths of 20,000 feet or more under conditions of high pressure and high temperature. Practices are limited to placing cement slurries in confined holes using the back pressure of the drilling fluid to control flow. Concrete is not used. Cement slurries are typically water, cement and various specialized admixtures. For certain purposes, such as increasing the unit weight of the grout, fine sand is sometimes used. The maximum sand grain size that can be accommodated by pumps and down-hole equipment is about 1/8-inch diameter. Sand, when used, is typically smaller than number 20 size; i.e., about 1/30-inch in diameter.
Well cementing methods have been adapted to some offshore platform construction: grouting platform pin-piles to the seafloor and grouting in anchor piles. On one occasion a number of bell-bottomed reinforced concrete piles of 31/2-foot diameter belled out to 9- to 15-feet diameter at the bottom end were constructed in a total depth of about 500 feet. A grout with maximum sand size of 1/30th of an inch was pumped into a drilled hole (which contained the steel reinforcing cage) to displace a weighted mud slurry.
Combined theoretical and empirical methods are used to predict the flow behavior in a pipeline of cement slurry treated as a non-Newtonian fluid. Flow calculations utilize experimentally determined coefficients related to slurry viscosity in laminar flow. This method is not directly applicable to plug flow of concrete in a pipe.
The major aspect of construction grouting and oil well cementing technology that is adaptable to deep ocean concrete placement is the control of material properties, particularly prevention of water loss from grouts and slurries under high pressures and pressure differentials. These properties are controlled primarily by careful selection of materials, control of mix proportions, control of procedures and use of specialized admixtures. Pumps and other equipment for grouting and cementing are not adaptable for concreting.
Mine Construction: Concrete for shaft and tunnel lining and other underground construction has been transported to the deep depths by dropping the freshly mixed concrete down long vertical pipes. Copper mines in the U.S. and gold mines in South Africa have shafts that are several thousand feed deep. The concrete segregates during the fall and is usually remixed at the bottom before being placed. This method is not applicable to underwater placement.
Slurry Transport: Particulate matter such as coal is transported long distances in "slurry pipelines." Similarly, spoil from hydraulic dredges and many materials in processing plants are transported in pipelines by two-phase flow with the suspended solid particles propelled by the drag forces of the faster moving water or other fluid. Usually turbulent flow is maintained to prevent particles from settling out. This technology is not applicable to pipeline transport of concrete.